1. Field of the Invention
This invention relates to semiconductor devices for transistors using silicon and to a method for manufacturing such devices.
2. Description of the Prior Art
Silicon whose purity is about 99,999% is used for photovoltaic devices (also known as solar cells) and also for transistors using semiconductor grade silicon.
For example, as to photovoltaic devices using semiconductor grade silicon, first, metallurgical grade silicon is prepared by reduction of ores such as silica sand. Then, a gas of the silane groups, such as monosilane, dichlorosilane and trichlorosilane, is made from such metallurgical grade silicon. Then, using these gases, a polycrystralline silicon or amorphous silicon (a-Si) which is semiconductor grade is produced.
To produce the above mentioned semiconductor grade silicon, additional refining treatments are needed from metallurgical grade silicon. These additional treatments increase the cost of the end product.
Indeed, when high purity silicon whose purity is over 99.99% is produced, the manufacturing costs drastically increase. Thus, the development of semiconductor devices using silicon whose purity is less than 99.99% is required.
In addition, prior art semiconductor devices such as photovoltaic devices, have p-n junctions produced by diffusion methods onto the wafer surface of high purity silicon whose purity is over 99.999%. As a result, the cost of raw materials is a large component of the manufacturing cost.
In addition to high costs, generally, the part of the device where the photovoltaic (photosensitive) function takes place is very near the wafer surface. Therefore, most of the wafer is merely used as a supporting substrate thus wasting material. Photovoltaic devices using such wafers are described in Bulletin of the Electrotechnical Laboratory, Vol.51, No.5,6 p378, 1987.